[mono] Don't throw inheritance error on interfaces in GetCustomAttrs

[CoffeeFlux]

Fixes#33639

Looking at #7190, the .NET Core behavior here is strange and my changes in 0844cb7a6152 with the type checking may have been a mistake. I think this should be fine with the interface special-casing, but if deemed too great a concern I can revert that subset of my old changes. Additionally, if people come up with other scenarios where this might fail, I'll just revert until we can migrate to use a shared CustomAttribute implementation rather than try to play whack-a-mole.

[CoffeeFlux]

@stephentoub can you check the test and make sure it's in the right place? Wasn't sure where to put it.

[akoeplinger]

I think I'd put it somewhere after

runtime/src/libraries/System.Runtime/tests/System/Attributes.cs

Line 204 in bd192ae

publicstaticvoidMultipleAttributesTest()

[CoffeeFlux]

From what I could tell, the GetCustomAttributes tests were all in the file for their parent class since their behavior differs some (e.g. MemberInfo tests have a bunch), which is why I put it here but wasn't sure. Can move if you still think that's the best place for it, and I'll fix the style.

[akoeplinger]

Can move if you still think that's the best place for it, [...]

I have no strong feeling on that one, the place you picked is probably fine too :)